Electrochemical cells exist in which the electrodes are separated by an ion-exchange membrane, for example a solid polymer electrolyte.
WO-A-03/23890 teaches that ion-exchange materials can be produced based on hydrophilic polymers, i.e. polymers which are inherently able to absorb and transmit water throughout their molecular structure. The materials are particularly relevant to hydrogen-oxygen fuel cells, since product water can be redistributed, thereby avoiding local flooding or drying-out of the membrane. There are essentially three ways in which a hydrophilic material can be rendered ionically conducting.
The first way is to form ionically active sites by co-polymerisation from a solution of ionically active monomers; this methodology is described in WO-A-03/23890. A strongly anionic or cationic moiety is formed in the resulting polymer, allowing it to function as a anionic-exchange (AE) or cationic-exchange (CE) material respectively.
The second way is to incorporate ionically active sites in the material by grafting ionically active monomers. An example of such a material is Nafion, which becomes ionically conducting when the ionic sites are activated by hydration in demineralised water.
The third way is by hydration of the hydrophilic material in an acidic or alkaline liquid. Hydrophilic materials such as HEMA (2-hydroxyethyl methacrylate) and MMA-VP (methyl methacrylate-vinylpyrrolidone) possess no intrinsic electrical properties and, if hydrated in deionised distilled (DD) water, are good electrical resistors. If, however, these materials are hydrated in an acidic or alkaline solution, they become good conductors. A limitation of this approach is that conductivity declines as the electrolyte solution washes out, the material eventually becoming electrically inactive. Such washing out is particularly problematic in electrolysers, where reactant water is normally present in large quantities, and in hydrogen-oxygen fuel cells, where water is produced.